This invention relates to the investigation of earth formations and, more particularly, to an apparatus and method for determining the amount of adsorbed or "bound" fluid in formations surrounding a borehole. The subject matter of this invention is related to subject matter disclosed in copending U.S. Application Ser. No. 674,792 of Rama Rau, filed of even date herewith and assigned to the same assignee.
Modern well logging techniques have advanced to a point where a number of subsurface parameters, for example porosity and lithology, can often be determined with reasonable accuracy. However, a reliable technique for determining the permeability of formations (i.e., a measure of the ease with which fluid can flow through a pore system), has not been forthcoming. Resistivity gradients have been used to estimate the order of magnitude of formation permeability, but this technique is found useful only in certain types of formations. It has been suggested that a measurement of the amount of "free fluid" in shaly sands would be a good permeability indicator. A known technique for measuring "free fluid" is the nuclear magnetic resonance tool, but attainable signal-to-noise ratios tend to be a limiting factor of performance for this tool.
Before discussing a new approach to the determination of the amount of free fluid in formations, it is helpful to discuss a recent development in the determination of dielectric properties of subsurface formations. In the past there were proposed various techniques for measuring the dielectric constant or electric permittivity of subsurface formations. Prior investigators recognized that the dielectric constant of the different constituents of earth formations vary widely (e.g., 2.2 for oil, 7.5 for limestone and 80 for water) and that the measurement of dielectric properties therefore holds promise of being a useful means of formation evaluation. However, prior art instruments for logging of dielectric properties of formations surrounding a borehole did not achieve hoped-for success for a variety of reasons. The dielectric constant of a lossy material can be expressed as a complex quantity of the form EQU .epsilon.* = .epsilon.' + j.epsilon."
The real part .epsilon.' in this equation represents the "true" dielectric constant of the material in lossless form; i.e., the measure of displacement currents for a particular electric field in the material if it were lossless. The imaginery part .epsilon." represents the dielectric "loss factor" of the material; i.e., the losses due to conduction and relaxation effects. Most previous efforts have been concerned with determining the value of .epsilon.' for a particular portion of subsurface formation. However, subsurface formation materials often have appreciable conductivity and thus a significant loss factor .epsilon." which is greater in magnitude than .epsilon.'. Since loss factor is necessarily measured to some extent when attempting to measure .epsilon.', the attainment of accurate values of .epsilon.' was until recently largely frustrated by the presence of a significant loss factor. An advance in this art is demonstrated in U.S. Pat. No. 3,849,721 as well as in U.S. Application Ser. No. 390,987 of R. Rau, now U.S. Pat. No. 3,944,910 assigned to the present assignee, which discloses an apparatus and method for determining dielectric properties of formations by injecting microwave electromagnetic energy into the formations and then taking measurements which determine the velocity of propagation of the microwave energy. In this technique the loss factor due to conductivity, which varies inversely with frequency, is kept small by employing relatively high frequency electromagnetic energy in the microwave portion of the spectrum. Also, the referenced application discloses a technique whereby a correction can be introduced which takes into account inaccuracies caused by conductivity.
The techniques described in the referenced patent and copending application are considered significant advances in the art, and it is one objective of the present invention to extend the types of techniques utilized therein to further advance the well logging art by enabling the obtainment of useful information about the amount of adsorbed fluid in subsurface formations as well as a further understanding of the relationships between "adsorbed" and "free" fluids in subsurface formations. Since the "total fluid" can be considered as the sum of the "free" fluid and the adsorbed (or "bound") fluid, and since total fluid can generally be determined from obtainable porosity and saturation information, if available, it will be understood that in references herein to determination of adsorbed fluid, it is implicit that free fluid could also be determined, if desired.
It is known that dielectric relaxation of ice occurs at radio frequencies and that the dielectric relaxation of free water occurs in the microwave region. Researchers have therefore suggested that loss measurements could be used to estimate the quantity of free water in a crystalline system. See, for example, "Progress In Dielectrics" Volume III, Edited by J. B. Birks and published by John Wiley Inc. (1961) and "Dielectric Relaxation of Surface Adsorbed Water", by Hoekstra and Doyle, Journal of Colliod and Interface Science, Vol. 36, No. 4 (1971). These measurements can be made in a laboratory but, since the dielectric relaxation frequency of free water is of the order of 10 GHz, there is difficulty in obtaining meaningful loss measurements at such high frequencies in a borehole environment where it is usually necessary to operate through a mudcake and the retrieving of signals at measurable amplitudes and reasonable signal-to-noise ratios if required.
Prior researchers have also speculated concerning the properties of adsorbed water (e.g. in the above references), and it has been suggested that adsorbed water has properties intermediate between ice and water and that dielectric spectroscopy could be a useful technique in elucidating the properties of adsorbed water. Some successful experimental work has apparently been performed along these lines in laboratory environments and in the study of, for example, ice/water/clay systems. However, the techniques employed to make laboratory measurements and the overall methods of approach do not lend themselves to obtaining useful information about subsurface formations surrounding a borehole.
In laboratory experiments it is possible to control various parameters in order to obtain information about one or more parameters under investigation. For example, in a prior art laboratory investigation of adsorbed or "bound" water in a particular system the effects of free water were virtually eliminated by freezing the system, and then a number of measurements were taken which reflected the effect of the bound water. Unfortunately, techniques of this type, as well as most laboratory techniques which involve control over the medium being measured, cannot be employed in a borehole environment. Also, practicalities of time in well logging applications limit the number of measurements that can be taken at each depth level.